The present invention relates to an indium gallium aluminum nitride-based edge-emitting laser diode structure and, more particularly, to a p-n tunnel junction for current injection for the indium gallium aluminum nitride edge-emitting nitride based laser diode structure.
Solid state lasers, also referred to as semiconductor lasers or laser diodes, are well known in the art. These devices generally consist of a planar multi-layered semiconductor structure having one or more active semiconductor layers bounded at their side ends by cleaved surfaces that act as mirrors. The semiconductor layers on one side of the active layer in the structure are doped with impurities so as to have an excess of mobile electrons. These layers with excess electrons are said to be n-type, i.e. negative. The semiconductor layers on the other side of the active layer in the structure are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. These layers with excess holes are said to be p-type, i.e. positive.
An electrical potential is applied through electrodes between the p-side and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers across the p-n junction so as to xe2x80x9cinjectxe2x80x9d them into the active layers, where electrons recombine with holes to produce light. Optical feedback is provided by the cleaved mirrors and a standing wave is formed between the mirrors in the laser resonator with a wave front parallel to the mirrors. If the optical gain produced in the active layers exceeds the optical loss in the laser structure amplified stimulated emission is produced and coherent laser light is emitted through the mirrored edges of the semiconductor laser structure.
Nitride based semiconductors, also known as group III nitride semiconductors or Group III-V nitride semiconductors, comprise elements selected from group II, such as Al, Ga and In, and the group V element N of the periodic table. The nitride based semiconductors can be binary compounds such as gallium nitride (GaN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium aluminum nitride (InGaN), and quarternary alloys such as indium gallium aluminum nitride (InGaAlN). These materials are deposited on substrates to produce layered semiconductor structures usable as light emitters for optoelectronic device applications. Nitride based semiconductors have the wide bandgap necessary for short-wavelength visible light emission in the green to blue to violet to the ultraviolet spectrum.
These materials are particularly suited for use in short-wavelength light emitting devices for several important reasons. Specifically, the InGaAlN system has a large bandgap covering the entire visible spectrum. III-V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under the high electric current and the intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown.
Semiconductor laser structures comprising nitride semiconductor layers grown on a sapphire substrate will emit light in the ultra-violet to visible spectrum within a range including 280 nm to 650 nm.
The shorter wavelength violet of nitride based semiconductor laser diodes provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) laser diodes for high-resolution or high-speed laser printing operations and high density optical storage. In addition, blue lasers can potentially be combined with existing red and green lasers to create projection displays and color film printers. The emission wavelength of GaN-based lasers and LEDs with an AlGaN or AlInGaN active region can be tuned into the UV range of the spectrum. Emission wavelength around 340 nm and 280 nm are particularly interesting for the optical excitation of biomolecules in bacteria, spores and viruses, which can be applied e.g. in bioagent detection systems.
A prior art nitride based semiconductor laser structure 100 of FIG. 1 has a sapphire (Al2O3) substrate 102 on which is epitaxially deposited a succession of semiconductor layers. The sapphire substrate 102 typically has a thickness of 200 micron to 1000 micron.
The prior art laser structure 100 includes an n-type III-V nitride nucleation layer 104 formed on the sapphire substrate 102. Typically, the buffer layer 104 is undoped GaN and has typically a thickness in the range between 10 nm and 30 nm.
A III-V nitride contact and current-spreading layer 106 is formed on the nucleation layer 104. The III-V nitride layer 106 is an n-type GaN:Silayer acting as a lateral n-contact and current spreading layer. The contact and current spreading layer 106 typically has a thickness of from about 1 xcexcm to about 20 xcexcm.
A III-V nitride cladding layer 108 is formed over the contact layer 106. The III-V nitride layer 106 is an n-type AlGaN:Si cladding layer. The cladding layer 106 typically has a thickness of from about 0.2 xcexcm to about 2 xcexcm.
On top of the III-V nitride cladding layer 108, a III-V nitride waveguide layer 110 is formed followed by the III-V nitride quantum well active region 112. The n-type GaN:Si waveguide layer 110 typically has a thickness of from about 50 nm to about 200 nm. The quantum well active region 112 is comprised of at least one InGaN quantum well. For multiple-quantum well active regions, the individual quantum wells typically have a thickness of from about 10 xc3x85 to about 100 xc3x85 and are separated by InGaN or GaN barrier layers which have typically a thickness of from about 10 xc3x85 to about 200 xc3x85.
A III-V nitride waveguide layer 114 is formed over the quantum well active region 112. The p-type GaN:Mg layer 114 serves as a waveguide layer and has a thickness of from about 50 nm to about 200 nm.
A III-V nitride cladding layer 116 is formed over the waveguide layer 114. The p-type AlGaN:Mg layer 116 serves as a cladding and current confinement layer. The III-V nitride cladding layer 116 typically has a thickness of from about 0.2 xcexcm to about 1 xcexcm.
A III-V nitride contact layer 118 is formed over the cladding layer 116. The p-type GaN:Mg layer 118 forms a p-contact layer for the minimum-resistance metal electrode to contact the p-side of the laser heterostructure 100. The III-V nitride contact layer 118 typically has a thickness of from about 10 nm to 200 nm.
The laser structure 100 can be fabricated by a technique such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy as is well known in the art.
Dry-etching using CAIBE (chemical assisted ion beam etching) or RIE (reactive ion beam etching) in an Ar/Cl2/BCl3 gas mixture is used to etch the prior art laser structure 100 down to the GaN:Si current-spreading layer 106.
An n-type Ti/Al electrode 120 is formed on the etched, exposed n-current-spreading layer 106 of the laser 100, which is functioning as a lateral contact layer. A p-type Ni/Au electrode 122 is formed on the p-contact layer 118 of the laser 100.
P-type doping of InGaAlN layers is a key problem in the realization of GaN-based devices. It is difficult to achieve a high hole concentration in AlGaN alloys since the ionization energy of Mg acceptors is relatively high (xcx9c200 meV for Mg in GaN) and increases even further with higher Al content (xcx9c3 meV per % Al). Therefore, p-doped waveguide and cladding layers contribute significantly to the series resistance of the nitride-based laser structure, which results in higher operating voltages. Even in today""s currently best violet nitride lasers, the operating voltages are on the order of 5 to 6 V, which is 2 to 3 V above the laser emission energy. For UV laser and LEDs, which require even higher Al compositions, the series resistance is going to be even larger. For a UV laser structure emitting around 340 nm the required Al composition for the cladding layers would be around 30%. The increase in Mg acceptor activation energy in the AGaN layer would result in an almost an order of magnitude drop in hole concentration compared to a Mg-doped GaN film.
In addition, the optimum growth temperatures for Mg-doped AlGaN layers is typically lower than the growth temperatures for Si-doped or un-doped AlGaN films, because of the improved Mg incorporation efficiency at lower temperatures. However, the structural quality of nitride-based semiconductor layers is reduced, when grown at a lower temperature, which deteriorates the structural and electronic properties of the upper cladding layers and upper waveguide layers in a III-V nitride laser structure.
Furthermore, in conventional InGaAlN laser diodes, GaN:Mg or InGaN:Mg are used as waveguiding layers and short period AlGaN/GaN supperlattice layers or bulk AlGaN layers doped with Mg are used as upper cladding layers. These Mg-doped layers have a significant absorption loss particularly in the blue to ultraviolet spectrum that a nitride based laser will emit light. For laser diodes operating close to the band gap of GaN ( less than 400 nm), this leads to increased distributed loss and consequently to increased threshold current densities.
It is an object of this invention to provide a nitride based semiconductor laser structure with a reduced number of p-type semiconductor layers.
According to the present invention, a p-n tunnel junction between a p-type semiconductor layer and a n-type semiconductor layer provides current injection for an edge-emitting nitride based semiconductor laser structure. The p-n tunnel junction reduces the number of p-type semiconductor layers in the nitride based semiconductor laser structure which reduces the distributed loss, reduces the threshold current densities, reduces the overall series resistance and improves the structural quality of the laser by allowing higher growth temperatures.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.